Modular and extensible lesson on optical fibre communication for youths

Optical fibre communication enables the global internet, but few youths ever learn about how it works, even at a basic level, until tertiary education. While some middle school curricula might include simple geometrical optics concepts like reflection and refraction, they often lack contextual linkage to worldwide telecommunications. Through our studies, we have found that students are more engaged in the learning process when the material directly relates to real life. To address this gap, and moreover to tackle the issue of the STEM skills shortage, we have designed a self-contained lesson to introduce youths to this topic. It is modularised into three parts, beginning with using light to communicate Morse code, and then covering advanced themes such as multiplexing and fibre guidance based on total internal reflection. The modules can be taught sequentially or individually depending on educational level. They emphasise a more phenomenological than theoretical approach and include hands-on activities using easily obtainable materials. We outline the lesson and pedagogical guidelines for classroom settings, as well as evaluate actual classes run. This lesson can be flexibly implemented in formal classes or through educational outreach programmes.


Introduction
In addressing skill shortages in science, technology, engineering, and mathematics (STEM) fields, studies have shown that life aspirations of youths typically begin to materialise before the age of 14 years, and thus suggest early intervention as a solution [1]. With appropriate tools and pedagogy, a seemingly advanced topic like fibre optics can appeal even to young students [2]. The active learning in optics and photonics (ALOP) framework [3] introduced by UNESCO emphasises an interactive and hands-on approach to learning, and has seen positive demonstrations at high school levels (ages 15+) [4,5].
Here, we apply ALOP principles at middle school levels (ages 11-14 years) and outline a lesson in optical fibre communication. It consists of three interactive modules covering different topics, each of which can be delivered alone or in succession depending on student level. Meant as an introduction, the modules assume no prior physics or engineering knowledge.
Module 1, which introduces encoding and basic communication using torchlights, can be taught to younger ages as a self-contained primer. For higher levels, Modules 2 and 3 cover multiplexing and refraction, respectively, and can function as individual or consecutive extensions or even standalone topics. Experimental exploration is encouraged and the modules use equipment that is easily obtainable, craftable, and hence replicable.
This lesson was first presented at the 2017 Conference on Education and Training in Optics and Photonics [6]. This paper extends the exposition and evaluates the module implementation in several class workshops.

Modules
To set the context, it is useful to introduce a communication system as a sequence of functional blocks depicted in figure 1 [7,8]. A message originates from a source and is encoded and sent by a transmitter as a modulated signal. The signal travels through a channel, which can take any form (e.g. optical fibre, copper cable, the atmosphere, etc.), to reach a receiver, which subsequently decodes the message for the destination. In multichannel systems, multiplexing (demultiplexing) occurs after (before) encoding (decoding) at the transmitter (receiver). As indicated, the coverage of the various modules relate to different parts of the system.
The following subsections outline each module, which includes some basic theory (as would be delivered to pupils) followed by one or more handson activities. Target ages are listed in parentheses.

Module 1: information (for ages 10+)
2.1.1. Coding and modulation. Information is represented as a sequence of binary bits, each of which can take one of two logical states. Computers use digital 1 and 0 to represent TRUE and FALSE, respectively. Two things are necessary to communicate information. Firstly, a system is needed to assign meaning to bit sequences. This is known as encoding. A well-known system probably familiar to youths is Morse code [9]. Here, the binary states are represented as dots • and dashes − , and each letter, numeral, and symbol is encoded as a distinct sequence of these. Table 1 lists the internationally used Morse codes. As an example, the universal distress signal 'SOS' is encoded as Secondly, a physical means of conveying the binary states is required. This can be conveniently actualised, in the optical context for example, by turning a torchlight on or off, or in Morse code proto col, by turning a torchlight on briefly (e.g. 1 s) for a dot and turning it on for longer (e.g. 3 s) for a dash [10]. In this way, a person sending a Morse coded message would be transmitting a sequence of long and short flashes. This is modulation. That is, the (encoded) message modulates the communication signal (e.g. light flashes) which then travels through a channel (e.g. the air) to be perceived by a receiver (e.g. another person seeing the flashes).

Class activity: Morse code.
In this activity, students work in pairs to send Morse coded messages to one another. Each pair is provided with a torchlight, a cardboard tube, a Morse code chart such as table 1, a Morse code decoding tree chart as shown in figure 2, pens or pencils, and some paper.
In each pair, one student is assigned as the sender and the other, the receiver. The sender decides on a message unbeknownst to the receiver. S/he encodes the message in Morse code referring to table 1, then transmits the encoding by turning the torchlight on and off accordingly and shines the beam through the cardboard tube, which acts as a fibre channel, as depicted in figure 3(a). The receiver observes and records the sequence of flashes from the tube's output. S/he then uses the dichotomic search chart in figure 2 to decode the message and verify it with the sender. For each encoded sequence, the receiver begins at start and descends the tree. Each dot or dash is a left or right traversal, respectively, to the next lower level. The node on which the traversal ends is the decoded character. For example, a 'D', encoded as − ••, has a traversal beginning at start and moving right→left→left. Although it is possible to use table 1 for decoding, the reverse lookup is unsorted and cumbersome. The chart in figure 2 is recommended as an easier alternative. After a round, the sender and receiver can swap roles if time permits. A number of tips can be considered. Firstly, the students should observe a no-speaking rule. This ties the activity closer to reality, where direct verbal communication is not possible over long distances. Secondly, situations where the receiver incorrectly decodes a message can be used as opportunities to introduce students to the existence of errors and their associated challenges. For instance, a receiver mistaking a dot for a dash is analogous to a bit error in digital systems. It is also useful to stress the importance of accurate clocks as integral components for robust bit discrimination. In this exercise, stopwatches could be used instead of having students mentally count seconds. Thirdly, students may discover that the manual process of sending dots and dashes is slow and tedious. This is a chance to highlight the need for ever faster transceiver systems, especially with exponentially growing internet traffic [11]. Lastly, it is helpful to let students interact with a simple home-built Morse code circuit, such as in figures 3(b) and (c), for them to appreciate how telegraphic Morse code was historically transmitted.

Module 2: multichannel systems (for ages 13+)
A useful question to pose to students is, 'how can multiple people each communicate using the limited fibre resources at the same time?' The answer is multiplexing, which is the transmission of information streams through a single medium by dividing it into multiple channels [8,10]. This Table 1. International Morse code chart, listing characters (left) and corresponding codes (right).  Figure 4 portrays a simplified WDM picture. Each sender, who may be communicating different types of data as illustrated by the various icons, uses a separate wavelength as indicated by the different coloured lines. These are all multiplexed (MUX) and transmitted simultaneously through the same fibre link. At the receiver side, the signals are demultiplexed (DEMUX) to separate each channel, and subsequently routed to the respective recipients.
One way to think of multiplexing is colour addition. Combining the primary colours, red, green, and blue, produces white [12]. So a white beam can be envisaged to contain all the information from the different coloured beams, as demonstrated in figure 5(a). It is the basis for energy-efficient white light-emitting diode (LED) sources, the contributions to which won the 2014 Nobel Prize in Physics [12]. Inversely, that white light is composed of the rainbow spectrum can be demonstrated via diffraction gratings, as in figure 5(b).

Class activity: wavelength-division demul-
tiplexing through filtering. Demultiplexing is the process where the individual channels that were multiplexed together are separated at the receiver [13]. Students learn about colour separation through filtering. They can be divided into small groups, with each group having a transparent container prepared with clear gelatin, a low-cost household material [14]; three other transparent containers, each with a different coloured jelly 2 ; and three slitted LED sources emitting red, green, or blue 3 .
In this activity, which was inspired by the 'Laser Jello' experiment from the Exploratorium R [15] and the 'Transmission with Gummy Bears' activity from LASER Classroom TM [16], students first shine all three LED colours into the clear gelatin, noting that they are all transmitted, as shown in figure 6(a). Thereafter, they experiment with the coloured jellies to observe them acting as wavelength (channel) filters that transmit only the like-coloured beams. Figures 6(b)-(d) depict this effect, where switching among the coloured jellies resembles the action of a tunable optical filter [8]. Laser pointers may be substituted for LEDs, though proper teacher supervision is advised and laser safety guidelines should be followed.
To make the experiment analogous to a fibre WDM system, the beams can be shone through a cascade of the clear gelatin followed by a coloured jelly, with the former representing a fibre link, as exemplified in figure 7(b). Likening the clear gelatin to an optical fibre and visually demonstrating all-colour transmission illustrates the fact that fibres can support WDM. While we acknowledge that this set-up does not exactly model actual multiplexed systems, since the beams are spatially separated, it nevertheless suffices as an analogy to real systems [17]. An alternative is to shine a (ray box) white beam into each coloured jelly and inspect the transmitted colour using a white screen placed after the jelly [18].
Worksheets asking which beams are transmitted through each coloured jelly, e.g. figure 7(a), can be used for students to record their findings.
It should be noted that because the jelly colours are not purely composed of the exact wavelengths of the corresponding beam colours, all beams are neither completely transmitted nor completely filtered out [19]. Rather, it is the relative degree of transmission and filtering that matters for this pedagogy. For instance, the red jelly in figure 6(b) mostly transmits the red beam and mostly filters out the green and blue beams. In figure 6(d), the purple jelly is actually closer in colour to the red beam than the blue one and therefore transmits red more.
For advanced students, a more rigorous explanation can be given. Each jelly has an absorption spectrum which determines the ranges of wavelengths that are absorbed or transmitted [20]. Such can be measured using a spectrophotometer as in [21], which shows the absorption for different flavours (colours) of jelly. For example, lemon and lime (green) jelly has low absorbance at green wavelengths (495 nm-570 nm) as expected, but only moderate absorbance at red wavelengths (620 nm-750 nm). Hence, some red light is transmitted.

Module 3: light guidance (for ages 14+)
Modules 1 and 2 concerned the transmitter and receiver. Following figure 1, here we explore the physics of how optical signals travel through the channel (i.e. the fibre). While the theory in sections 2.3.1 and 2.3.2 is didactically sufficient, the reader is referred to most physics or optics textbooks, e.g. [8,10,13,22], for more details.

Reflection and refraction.
Whereas reflection, e.g. in bathroom mirrors, should be a recognisable concept, students may be less familiar with refraction and its physics. The perceived bending of a straw in water as in figure 8(a) is a real-world example.
Refraction is explained as a change in the speed of light as it travels between materials of different refractive indices. For young students, it is unnecessary (and inadvisable) to elaborate on the microscopic particle interactions causing this change. Rather, it suffices to mention that light slows down when travelling through an optically denser material, which has a higher refractive index. As shown in figure 8(b), this change in the light's propagation speed causes it to bend at the interface. The indices n are related to the incidence (θ 1 ) and refraction (θ 2 ) angles of light by Snell's law of refraction [10]:   figure 9a shows light travelling from a higher to a lower refractive index, with θ 1 < θ 2 since light travels faster in n 2 . As θ 1 is increased, so too will θ 2 until it is perpendicular to the normal, as in figure 9(b). Here, the incidence angle is known as the critical angle θ c . If θ 1 is increased beyond θ c as in figure 9(c), then no light is refracted and all light is instead internally reflected [13]. The interface resembles a mirror and the reflection angle θ 2 is equal to θ 1 . This is total internal reflection (TIR) [22]. It is emphasised that TIR can only occur for light travelling from within a material of higher refractive index to one of lower refractive index. Light guidance in optical fibres is possible via multiple longitudinal TIRs [8].

Class activity: measurement of refrac-
tive index. The objective is to measure the refractive index of gelatin. Students can work in groups, each given a transparent square container with clear gelatin as per section 2.2.2, a 360° protractor, a slitted LED source, a scientific calculator, pens or pencils, and an associated worksheet. The experiment is set up with light passing between air and gelatin. From (1), the unknown refractive index of gelatin n gel can be determined from the known index of air (n air ≈ 1) as well as the angles of incidence θ air and refraction θ gel which are to be measured. This is done by aligning the container's edge to the protractor's 0°-180° axis. A narrow LED beam is shone angularly through the edge of the container, at the protractor's origin, as depicted in figure 10(a). n gel > n air causes the light to slow down and refract through the gelatin. By measuring θ air and θ gel , and rearranging (1), n gel can be calculated via Depending on whether the students are familiar with trigonometry, it may be necessary to assist them with calculating the sines of angles using the scientific calculator.
A simple extension to the activity is for the students to measure the refractive index of water, as pictured in figure 10(b). The indices of water (∼1.33) and gelatin (∼1.34-1.40) can then be compared and a discussion can follow about how gelatin, being a solid, has an index closer to that of the glass in optical fibres (∼1.45 [8]). Although any LED colour can be used for the measurements, it is important to use the same colour for both the gelatin and water cases, since different colours (wavelengths) travel at different non-vacuum speeds [10,22] and will refract at slightly different angles.

Class demonstration: light guidance using
TIR. Continuing from the refraction topic, the effect of TIR as discussed in section 2.3.2 is explored. Either the students can examine this themselves by manipulating the incidence angle on the gelatin set-up as in figure 11(a), or a teacher can demonstrate the Tyndall/Colladon experiment [23], where laser light is guided through a water stream as in figure 11(b). The latter requires a large transparent water-filled tank with a faucet, a laser pointer, and a water collection bucket. A combination of both demonstrations is encouraged.
An additional useful teaching tool is a fibre fountain, shown in figure 11(c). The hairlike plastic strands provide a safe emulation of real glass fibres.

Evaluation and discussion
The modules were implemented in formal classes, delivered by graduate student demonstrators through our university's Lightwave Roadshow educational outreach programme [23]. For each module, a set of learning objectives was identified, and worksheets or quizzes were used to assess the quality of learning and the practical skills demonstrated. Surveys were also conducted to gauge enjoyment and attitudinal changes, using the methodology in [24]. With reference to table 2, this is a summary of activities undertaken: They earned 1 mark for making an observation, 1 mark for noting selective filtering, and an additional 1 2 mark for further comments about imperfect filtering (e.g. green jelly not completely blocking red and blue).
• Module 3 was implemented in two separate sittings; in a combined lesson with Module 2 (previous bullet), and on its own with 9 11-16-year-olds. Both sittings included the gelatin index measurement exercise. In the latter, it was observed that the younger students (11-14 years) required extra guidance working with Snell's law and trigonometric calculations. Nevertheless, all were able to complete the exercise. Students scored 1 mark for angle measurements, 1 mark for correct application of (2), and 1 2 mark if their answer was within the reasonable range.
The average scores in table 2 were computed via and indicate strong achievement of the learning objectives. Determination of appropriate age ranges was also informed by demonstrator observations and student feedback. Particularly, some 14-year-olds doing Module 2 found the filtering activity rather easy. Thus in relation to our previous paper [6], we have reordered Modules 2 and 3 and lowered the target age of Module 2 (previously ages 16+) 4 . In classes covering Modules 2 and/or 3, students were asked, both before and after the classes, about their likelihood of recommending studying photonics or electronics. As figure 12 shows, whereas most 'pre' responses were predominantly negative, reflecting stereotypical STEM perceptions, there were consistent favourable shifts in attitude post-class.

Conclusions and future work
We have outlined a modular hands-on lesson covering en/decoding, multiplexing, and fibre guidance. While the modules, designed for primary levels and up, can be taught individually (within 30-minute sessions) or in succession, they all serve to introduce youths to optical fibre communication and spark interest in STEM. The practical mat erials required are cheap, easily sourced, and can be prepared or built at home. Future improvements can include adding modules on other topics, such as polarisation, as well as adding more challenging concepts to tailor earlier modules for older ages, e.g. diffraction for Module 2. More evaluation data can also be obtained, including for running all modules consecutively.
It is hoped that this lesson may be helpful to other educators.

Matthew T Posner is a Process
Scientist at Excelitas Technologies, Montreal, QC, Canada, specialising in diffusion, oxidation, and chemical vapour deposition processes in silicon devices. He obtained an MEng in electronic engineering from the University of Southampton in 2013 and a PhD from the Optoelectronics Research Centre, University of Southampton in 2017. Dr Posner's doctoral research was in the subject area of photonic integrated circuits and planar Bragg gratings for applications in quantum science and sensing. Dr Posner is an active science communicator and is passionate about "getting the science out of the lab". He is a member of the OSA, SPIE, and IEEE Photonics Society, through which he supports outreach education and public engagement initiatives. He has contributed to raising close to £40,000 for outreach education and public engagement programs. He has received awards for his endeavours, with the most notable featuring the 2017 University of Southampton Students' Union's Award for Best Contribution to a Society and the 2015 South East England Physics Network Public Engagement Innovation Award.